Hydric soils form as the result of chemical reactions influenced by geological and biological processes. The presence of water is the most prominent geological function that leads to the evolution of these soil types. Conversely, anaerobic conditions caused when oxygen is unable to penetrate through the soil due to saturation, coupled with biological responses, elicit chemical reactions in the soil. These chemical reactions leave behind observable and/or physical features known as hydric soil indicators. The reduction and oxidation of iron oxides in the soil profile are the most prominent and observable processes when identifying hydric soil indicators. The process starts when organic matter, such as dead roots and leaf litter, undergo decomposition as bacteria use this as a food source. When this process occurs under waterlogged conditions, iron compounds in the soil lose oxygen molecules as the bacteria continue to feed on the organic material. The iron then loses the oxygen it was bonded to in the soil, causing it to appear grey, as opposed to the yellow or red hues seen under normal soil conditions. When the soil is no longer saturated and oxygen is able to reenter, the iron is then able to bond with incoming oxygen molecules, causing reddish masses, or mottles, to form along root channels or wherever the iron accumulated during the previously saturated conditions. The grey layers of soil caused by the chemical reactions and movement of iron are called depleted layers. To tell if a soil layer is depleted, a Munsell Soils Color Guide is used to identify the soil color. These grey colors are identified by a chroma of 2 or less and value 4 or more in the Munsell guide. In addition to meeting the definition of a depleted layer, the soil must sometimes contain reddish concentrations that result from the iron reduction and oxidation processes discussed above. While iron reduction is not the only chemical reaction taking place under anaerobic and saturated conditions, it is arguably the most prominent and often observed process when attempting to identify hydric soil features. Other chemical compounds that undergo similar processes include manganese oxides and sulfates.
Not all hydric soils will exhibit characteristics associated with iron reduction. However, most hydric soils share similar attributes regarding their permeability and drainage capability. These soils range from moderate to slow in permeability and are somewhat poorly drained to very poorly drained. Based on these drainage flow rates, surface water ponding is typical but not necessary. Subsurface soil saturation and/or a high water table can also cause hydric soil indicators to form. Hydrology plays an integral part in either scenario and is necessary in the formation of soil indicators. Due to the relatively flat nature of the lowcountry landscape, soil permeability and drainage rates provide helpful clues when assessing soil conditions. Even soils listed as non-hydric by the NTCHS can meet certain indicators for hydric soils, based on these
Hydric soils and their indicators can be difficult to identify, as there are 41 unique indicators listed in the Regional Supplement for the Atlantic and Gulf Coastal Plain Region. Many of these indicators are driven by chemical reactions involving iron compounds in the upper 20 inches of the soil. The presence of anaerobic and saturated conditions drives the creation of hydric soil indicators. Soil texture, morphology, and landscape position also have roles to play in their presence. As the USACE continues to rely more heavily on the presence of hydric soils when making wetland determinations, the need for soil scientists and consultants that can accurately identify hydric soil indicators will continue to increase.
Environmental Laboratory. (1987). "Corps of Engineers Wetlands Delineation Manual," Technical Report Y-87-1, U.S. Army Engineer Waterways Experiment Station, Vicksburg, Miss.
U.S. Army Corps of Engineers. 2010. Regional Supplement to the Corps of Engineers Wetland Delineation Manual: Atlantic and Gulf Coastal Plain Region (Version 2.0), ed. J. S. Wakeley, R. W. Lichvar, and C. V. Noble. ERDC/EL TR-10-20. Vicksburg, MS: U.S. Army Engineer Research and Development Center.
United States Department of Agriculture, Natural Resources Conservation Service. 2010. Field Indicators of Hydric Soils in the United States, Version 7.0. L.M. Vasilas, G.W. Hurt, and C.V. Noble (eds.). USDA, NRCS, in cooperation with the National Technical Committee for Hydric Soils.
Hydric Soils in the Lowcountry